In recent years, mobile communication systems such as a PHS (Personal Handy phone System) have been rapidly developed, and these systems have employed a TDMA method, in which one frame (5 milliseconds) formed of four slots (625 microseconds per slot) is used as a basic unit for transmission and reception. This TDMA method of the PHS has been standardized, e.g., as a “second generation cordless communication system).
In the communication system of the PHS, a signal in one frame is divided into eight slots, which are divided into a first half including four slots for reception as well as a latter half including four slots for transmission.
Each slot is formed of 120 symbols. For example, the signal in one frame is handled such that three slot sets each formed of one reception slot and one transmission slot are allocated to traffic channels for three users, respectively, and remaining one set of slots is allocated to a control channel.
According to controlling procedures for establishing synchronization in the PHS, a link channel is first established by a control channel, and processing of measuring interference wave (U-waves: Undesired waves) is performed. Further processing is performed to set communication conditions with allocated channels, and thereafter, the speech communication starts. The above procedures are disclosed in detail by standards of the PHS, and particularly by the second generation cordless communication system standards RCR STD-28 issued by Association of Radio Industries and Businesses.
FIG. 21 illustrates a communication sequence flow of the PHS. Referring to FIG. 21, this flow will now be briefly described.
By using a C-channel (Control channel: CCH), a link channel establishment request signal (LCH establishment request signal) is sent from a PHS terminal to a base station. The PHS base station detects an empty channel (i.e., empty traffic channel (empty T-channel)), and sends a link channel allocation signal (LCH allocation signal) to the PHS terminal side via the C-channel.
On the PHS terminal side, it is determined based on link channel information received from the PHS base station whether interference wave signals of a predetermined power or higher are received or not on a designated T-channel, and thus U-wave measurement and carrier sense are performed. When interference wave signals of the predetermined power or higher are not detected, i.e., when another PHS base station is not using the designated T-channel, a synchronous burst signal is sent to the base station by the designated T-channel, and a synchronous burst signal is also returned from the base station to the terminal to establish the synchronization.
When interference wave signals of the predetermined power or higher are detected on the designated T-channel, i.e., when another PHS base station is using the designated T-channel, the PHS terminal repeats the control procedures starting from issuance of the request signal for the link channel establishment.
As described above, the PHS connects the communication channel between the terminal and the base station by using the channel, which can provide good communication characteristics suppressing the interference waves.
For maintaining a good communication quality by suppressing an influence by communication of another base station, the PHS may perform transmission and reception with directivity when the base station transmits a signal to the terminal or receives a signal therefrom.
In the PHS and others, a PDMA (Path Division Multiple Access) system is available for increasing an efficiency of use of wave frequencies. In the PDMA, spatial multiple connection can be achieved between mobile radio terminal devices (terminals) of a plurality of users and the radio base station (base station) via a plurality of paths, which are formed by spatially dividing the same time slot of the same frequency.
For example, an adaptive array technology has been employed for achieving the directivity in the transmission and reception and achieving the PDMA system described above. The adaptive array processing can accurately extracts signals from a desired terminal by adaptive control, which is performed by calculating a weight vector (receive weight vector) formed of receive coefficients (weights) for respective antennas of the base station based on the signal received from the terminal, and thus, by multiplying the reception signals of the plurality of antennas by respective elements of the receive weight vector.
By the adaptive array processing, an uplink signal sent from the antenna of each user terminal is received by the array antenna of the base station, and the signal thus received is separated and extracted with a receive directivity.
The transmission signals (i.e., signals to be sent) are processed such that signals, which are produced by multiplying the transmission signal by respective elements of the transmission weight vector calculated from the receive weight vector, are transmitted from the plurality of antennas, and thereby a downlink signal to be sent from the base station to the terminal is transmitted with a send directivity with respect to the antenna of the terminal.
The above adaptive array processing is well known, and is specifically disclosed in Nobuyoshi Kikuma “Chapter 3: MMSE Adaptive Array” in “Adaptive Signal Processing by Array Antenna”, Kagaku Gijutsu Shuppan, Nov. 25, 1998, pp. 35-49. Therefore, an operation principle thereof is not described in this specification.
Among the signals received in the PDMA system, a desired signal is identified in the following manner. A radio signal transmitted between a terminal such as a cellular phone and a base station is divided into a plurality of frames, and thus has a so-called frame structure when it is transmitted. Each frame includes eight slots formed of, e.g., four slots for uplink communication and four slots for downlink communication. This slot signal is basically formed of a preamble formed of a signal series, which is known on a receiver side, and data (such as voice) formed of a signal series, which is unknown on the receiver side.
The preamble signal series includes a signal series (reference signal such as a unique word signal) of information for determining whether the sender on the other end is the desired party for the receiver side or not. For example, the adaptive array radio base station performs weight vector control (determination of weight coefficients) to extract a signal, which is presumed to include a signal series corresponding to the desired party on the other end, based on a comparison between the unique word signal obtained from the memory and the received signal series.
Further, each frame includes the foregoing unique word signal (reference signal) section, and is further configured to allow error detection with cyclic codes (CRC: Cyclic Redundancy Check).
FIG. 22 is a conceptual view illustrating a state of communication between a conventional radio device and a base station.
In an example illustrated in FIG. 22, the communication is being performed between a radio base station CS1 and a radio terminal device PS1, and the communication is also being performed between another radio base station CS2 neighboring to radio base station CS1 and another radio terminal device PS2.
In FIG. 22, it is assumed that non-directional transmission and reception is performed for both the communication from the radio terminal device to the base station, which will be referred to as “uplink communication” hereinafter, and the communication from the radio base station to the radio terminal device, which will be referred to as “downlink communication” hereinafter.
In the structure illustrated in FIG. 22, a problem of interference is liable to occur when the communication in a peripheral cell, which is a communication region of radio base station CS2, is being performed with the same frequency and at the same time as the communication in a cell, which is a communication region of radio base station CS1.
More specifically, in the uplink communication, the quality of the uplink communication in the station's own cell is impaired by the interference from the peripheral cell.
Likewise, in the downlink communication, the interference is applied to the peripheral cell to impair the downlink quality of the terminal, which is communicating in the peripheral cell.
In FIG. 22, arrows with dotted lines indicate interference signals occurring between radio base station CS1 and radio terminal device PS2.
For suppressing the interference during the simultaneous communication with the same frequency, the adaptive array transmission and reception already described may be performed.
However, if the adaptive array transmission and reception is performed in both the uplink communication and the downlink communication, this increases an installation cost of the base station.
More specifically, for performing the adaptive array processing in the downlink communication, an expensive high-power amplifier is required for every antenna of the base station, resulting in the high installation cost.
Further, each base station requires a signal processing circuit for controlling the downlink directivity, which further increases the cost.
If the FDD (Frequency Division Duplex) system is employed, it is difficult to control accurately the directivity.
In other words, a difference occurs in amount of rotation of a phase, which is caused when passing through a propagation path, between the uplink communication and the downlink communication. Therefore, it is difficult to control the directivity in the downlink communication based on information related to the directivity obtained in the uplink communication so that accurate control of the directivity is difficult, and interference with the peripheral cell increases.
The invention has been developed for overcoming the above problems, and it is an object of the invention to provide a radio device, a transmission and reception directivity control method and a transmission and reception directivity control program, which can reduce an installation cost of the base station, and can suppress interference signals affecting a peripheral cell.